The present invention relates to a process and equipment for the production of high-purity polycrystalline silicon in rod form for semiconductor applications. The polycrystalline silicon is used as the raw material in the fabrication of single crystal silicon for semiconductors by the CZ (Czochralski) method or the FZ (float zone) method.
The most common method of producing polycrystalline silicon, which is a raw material used for the production of single crystal silicon for semiconductors, has been to deposit silicon on starter filaments by thermal decomposition of a halosilane compound, such as trichlorosilane, so as to produce large-diameter silicon rods. Japanese Patent Laid-Open No. 56-105622 discloses a reactor structure using a chloride-type silane in which a large number of electrodes are arranged on a circular plate and a large number of silicon starter filaments are arranged in a reverse-U-shaped or a square-reverse-U-shaped form.
This technique, however, is not suitable for industrial scale production of polycrystalline silicon from a silane compound, such as monosilane gas or disilane gas, which is not halogenated. At a temperature of several hundred degrees or more, monosilane gas decomposes and thereby generates a fine silicon powder. The presence of such powder causes a number of difficulties and, in particular, can seriously hinder the growth of silicon rods. Further, where the high-temperature silicon rods face each other, surface irregularities are generated, thereby deteriorating product quality.
A known technique for dealing with the above problems is disclosed in U.S. Pat. No. 4,150,168, according to which red-hot silicon starter filaments are thermally insulated from each other so as to prevent vapor-phase temperature rise and as to eliminate thermal influences from the adjacent heated silicon rods, thereby obtaining uniform silicon rods.
However, in the industrial scale production of silicon rods by thermal decomposition of monosilane, it is impossible, even with the above-mentioned technique, to reduce the silicon powder generation to zero. The generated silicon powder is deposited on the reactor walls. When it has accumulated to a thickness of several mm, the silicon powder spontaneously separates from the walls and falls, part of the falling powder contacting and adhering to the growing silicon rods. The portion of the powder which adheres to the silicon rods may lead to powder intrusion, abnormal dendrite growth or the like, resulting in defective products.
Japanese Patent Laid-Open No. 61-101410 discloses a technique which is somewhat improved over that of U.S. Pat. No. 4,150,168, in that the reactor has a different heat insulation structure. However, for reasons given in a reference by Hogness et al. (Hogness, T. R., Wilson, T. L., Johnson, W. C.: "The Thermal Decomposition of Silane", J. Am. Chem. Soc. 58: 108-112, 1936), the new technique is likely to require a serious decrease in reaction speed in order to obtain the restraint of the silicon powder growth.
Japanese Patent Publication No. 44-31717 discloses a technique for collecting silicon powder outside a reactor. With this technique, the silicon powder generated in the course of production of polycrystalline silicon rods is taken out of the reactor along with the partially spent reactant gas. The powder is collected by means of a filter, and the gas cleared of powder is re-circulated through the reactor. A similar technique is disclosed in U.S. Pat. No. 4,831,964. A problem with these techniques is that they require large scale equipment external to the reactor. Thus, they involve an increase in the number of components, resulting in an increase in the opportunity of contamination. Further, the silicon powder adhering to such components accumulates in places where it cannot be easily removed by cleaning or in places which are hard to clean. The silicon powder is very active, so that it is easily ignited by static electricity or the like. And, an ignition of a mixture of air and silicon powder can cause a detonation. It is another problem that silicon powder deteriorates the sealing property of valves used to isolate the reactor from the external equipment when extracting silicon rods, performing cleaning, etc. Thus, handling of the silicon powder is best kept to a minimum.
Japanese Patent Publication No. 52-36490 discloses a special method of causing a reactive gas to circulate in a reactor. The method employs a means for uniformalizing the concentration of monosilane gas in the reactor. It prevents monosilane gas at high concentration or pure monosilane gas from reaching a high-temperature section of the reactor in the vicinity of the silicon starter filaments, thereby restraining the generation of silicon powder. A problem with this method is that no measure is taken to contain the radiation of heat from the heat generating elements. Thus, the technique is not suitable for the thermal decomposition of monosilane gas. Further, because the rods are not grown in separate reaction chambers, it is difficult to supply the reactive gas in a uniform fashion. As a result, it is hard for the grown silicon rods to attain a high level of roundness in cross section, the rod diameter differing from rod to rod.
A technique for increasing the flow velocity of reactive gas is disclosed in Japanese Patent Laid-Open No. 63-123806, according to which an agitator is provided in the top or bottom section of a reactor. This technique, however, is not suitable where a nonhalogenated silane compound gas is used since silicon powder would be generated and dispersed by the agitator.
Apart from the problems discussed above, these prior techniques have a problem which is common to them: the absence of a means for preventing the silicon powder which is generated by vapor-phase homogeneous reaction, from accumulating on the walls around the silicon rods and on the reactor ceiling. Defective products result due to the adhesion of silicon powder detached from the reactor walls. Silicon rods are hard to dissolve where the silicon powder has adhered, thus making monocrystallization difficult. Therefore, silicon rods with adhered powder are suitable for neither the CZ or the FZ method.
Further, it is considered that the precipitation rate of polycrystalline silicon will be low when a reactor structure encourages the accumulation of silicon powder on the walls around growing polycrystalline silicon rods and on walls in the reactor ceiling section.
Polycrystalline silicon, in the form of rods or chunks obtained by crushing rods, is being widely used in the production of single crystal silicon by the CZ or FZ method. A high purity level and competitive cost are particularly required of polycrystalline silicon rods for semiconductor applications. These requirements are becoming severer from year to year. The present invention has been made in view of the above problems in the prior art.
Accordingly, there is a need to provide a process and equipment which make it possible to produce large diameter polycrystalline silicon rods rapidly while making efficient use of a gas feedstock that contains a nonhalogenated silane compound.